For the purposes of telecommunication, national and international bodies assign frequency bands (or channels) within the radio spectrum for specific uses, and in most cases, license the rights to these channels. If the spectrum is not being used by licensed services in a specific location at a specific time, unlicensed radio transmitters are allowed to operate in such spectrum at such locations. This part of the spectrum is often termed “white-space”.
As a result of the digital switchover currently taking place around the world, bandwidth formerly used for broadcasting analogue television signals is being made available for other use. This is possible because digital transmissions can be compressed into fewer channels than analogue transmissions. In the United States, these abandoned television frequencies are primarily in the upper UHF (700 MHz) band, covering channels 52 to 69 (609-806 MHz). The use of white-space in other countries/regions will depend on the local regulatory authorities.
Various proposals have advocated using white spaces left by the digital switchover to provide wireless broadband internet access. As TV frequencies propagate over long distances and penetrate through obstacles, this part of the spectrum is considered to be ideal for such use.
On the 4 Nov. 2008, the Federal Communications Commission (FCC) approved the unlicensed use of white space in the United States provided certain rules are strictly abided by in order to avoid the potential interference issues. Firstly, white-space devices will have to include geographical location technology to match the current device location against a pre-existing database of spectrum users. All unlicensed white-space devices must therefore access the database to determine permitted channels before operating, and fixed devices must register information concerning the location of their operations in the database. Used together, the geographical location and database access features should allow fixed and portable white space devices to detect whether they are in danger of interfering with other signals and redirect their transmissions accordingly, possibly ceasing operation where necessary. Finally, the FCC also requires new unlicensed white-space devices to include spectrum-sensing technology allowing them to detect the presence of other signals in their vicinity.
On start-up, white-space devices will query the database with their current location (likely gleaned from GPS or mobile telephone triangulation) and will receive in response a list of frequencies that can (and/or cannot) be used within their local area. The geo-specific spectrum usage data received from the database may then be stored on the memory of the device. For portable white-space devices, the stored spectrum usage data must be updated to coincide with the changing location of the device. The best approach for updating the device with spectrum usage data is therefore an issue. Important considerations include the frequency of updates as well as the apparatus and method required to perform these updates.
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